Someone posted about Brooks C17 saddles, but unfortunately I deleted
the post before I got around to answer it.

There seem to be several versions of the "Cambium" saddles, C17, C17s
and C15 which seem to be intended to match the size and shape of the B
series saddles.

Did you find that if, for instance a B17 fitted that the C17 also
fitted?

The shape is closer to the newer B.17s rather than the 20+ yer old ones,
which is what my B.17 is. But the C.17 is very close to my 70s Pro,
which is the most comfortable saddle I have ever had (well, after it
broke in).
And second, I had read that the C series took some breaking in, not as
much as the leather saddles, but some. Did you find that true?

Nope, good right out of the box. It's a thick rubber base with a
fabric top bonded to it, so there's really nothing that can break in.
Like all Brooks saddles, though, comfort is very sensitive to saddle
angle.